convert: allow the converter_source to say "skip this revision"

If getchanges returns a string, it's assumed to be the id of an
already converted revision.  We map the current revision to the same
revision this converted revision was mapped to.

To allow skipping a root revision, getchanges can return the special
string 'hg-convert-skipped-revision' (a.k.a. common.SKIPREV), which
hopefully won't clash with any real id.

The converter_source is responsible for rewriting the parents of the
commit objects to make sure the revision graph makes sense.

hgext/convert/__init__.py

@@ -5,7 +5,7 b''
 # This software may be used and distributed according to the terms
 # of the GNU General Public License, incorporated herein by reference.
 
-from common import NoRepo, converter_source, converter_sink
+from common import NoRepo, SKIPREV, converter_source, converter_sink
 from cvs import convert_cvs
 from darcs import darcs_source
 from git import convert_git
@@ -202,7 +202,15 b' class converter(object):'
         do_copies = hasattr(self.dest, 'copyfile')
         filenames = []
 
-        files, copies = self.source.getchanges(rev)
+        changes = self.source.getchanges(rev)
+        if isinstance(changes, basestring):
+            if changes == SKIPREV:
+                dest = SKIPREV
+            else:
+                dest = self.map[changes]
+            self.mapentry(rev, dest)
+            return
+        files, copies = changes
         parents = [self.map[r] for r in commit.parents]
         if commit.parents:
             prev = commit.parents[0]
@@ -263,7 +271,7 b' class converter(object):'
             ctags = {}
             for k in tags:
                 v = tags[k]
-                if v in self.map:
+                if self.map.get(v, SKIPREV) != SKIPREV:
                     ctags[k] = self.map[v]
 
             if c and ctags:

hgext/convert/common.py

@@ -17,6 +17,8 b' def decodeargs(s):'
 
 class NoRepo(Exception): pass
 
+SKIPREV = 'hg-convert-skipped-revision'
+
 class commit(object):
     def __init__(self, author, date, desc, parents, branch=None, rev=None):
         self.author = author